南海トラフ巨大地震は、フィリピン海プレートとアムールプレートとのプレート境界の沈み込み帯である南海トラフ沿いが震源域と考えられている巨大地震です。時には超巨大地震となることもあります。この地震は南海トラフ沿いの巨大地震とも呼ばれています。
南海トラフの地震は約90 - 150年（中世以前の発生記録では200年以上）の間隔で発生し、東海地震、東南海地震、南海地震の震源域が毎回数時間から数年の期間をおいてあるいは時間を置かずに同時に3つの地震が連動していることが定説です。最大クラスの南海トラフ巨大地震（Mw 9.1）が発生すると、超広域にわたる巨大な津波や強い揺れが発生し、西日本を中心に大規模な人的・物的被害が発生し、国全体に深刻な影響を与える国難とも言える巨大災害になることが想定されています。
南海トラフにおける地震の震源域は毎回異なり、地震が他の地震に誘発される場合があるため、発生時期が誘発で拘束されるため時間予測モデルは成立しない可能性も指摘されています。ただし、南海トラフ沿いの巨大地震は「再帰性」と複数の固有地震の震源域で同時に起こる「連動性」が大きな特徴となっており、その発生確率を評価するための様々なモデルや研究が行われています。
南海トラフ巨大地震の発生確率については、時間予測モデルや発生間隔のみを評価する方法などがあります。時間予測モデルを用いる場合、高知県室戸市室津漁港の隆起量や室津港の歴代南海地震における隆起量と発生間隔などを元にして、発生確率を計算する試みも行われています。ただし、これらのモデルは不完全であるとされ、異論も存在します。
具体的な数値としては、2013年時点では南海トラフでM8 - 9クラスの地震が発生する確率が約60% - 70%程度とされていましたが、2018年時点では70% - 80%程度と評価されています。また、最大クラス（M9超）の地震が発生する可能性は、100 - 200年間隔で発生している地震に比べて非常に低いとされています。
発生間隔のみを評価する場合、異論のある1605年慶長地震を含むか否か、また他の地震を含むかどうかによって平均発生間隔は異なり、評価が難しい状況となっています。発生確率も平均活動間隔に基づいて計算されており、その結果、最尤法で算出された30年以内の発生確率は10%程度から30%程度まで幅広く変動しています。
ただし、これらの予測やモデルはすべて予測であり、地震の発生は予測が難しい自然現象であるため、これらを根拠にした具体的な予測は難しいとされています。南海トラフ巨大地震に備えるためには、防災・減災対策が重要であり、社会全体での備えが求められています。
The Nankai Trough Megathrust Earthquake is considered a massive earthquake that occurs along the subduction zone of the boundary between the Philippine Sea Plate and the Amur Plate. It is sometimes referred to as a super megathrust earthquake. The specific term used is "Giant Earthquake along the Nankai Trough" (Nankai Trough Zyokai Jishin) [6][7][8].
The occurrence years and epicentral regions of past Nankai Trough earthquakes are as follows:
- Keicho Earthquake (1605)
- Hoei Earthquake (1707)
- Ansei Earthquake (1854)
- Taisho Earthquake (1944)
- Showa Earthquake (1946)
A "Giant Earthquake of the Nankai Trough" is a term used to describe the anticipated maximum-class earthquake along the Nankai Trough, as discussed by the "Giant Earthquake Model Study Committee for the Nankai Trough" established by the Cabinet Office in August 2011 [9][10].
Characteristics of Nankai Trough Earthquakes and "Earthquake Scenarios": The potential impact of a magnitude 9.1 Nankai Trough Megathrust Earthquake is anticipated to cause extensive damage, including massive tsunamis and strong shaking, primarily affecting western Japan. The resulting human and material losses are expected to surpass those of the Great East Japan Earthquake, significantly affecting the national life and economic activities, leading to a colossal disaster [11].
The Nankai Trough earthquakes typically occur at intervals of approximately 90 to 150 years, with a well-established pattern of three interconnected earthquakes—Tokai Earthquake, Tonankai Earthquake, and Nankai Earthquake—happening either simultaneously or within a few hours to several years. However, opinions on the recurrence intervals vary, and there are debates regarding the seismic activity in the Nankai Trough before the Keicho Earthquake in 1605 [12].
Recent studies have revealed that the earthquake source areas vary slightly with each occurrence. The earthquakes are classified into two types: the "Ansei-type," where the rupture of the fault progresses from the eastern end of the Nankai Trough (part of Tonankai and Tokai) to Shizuoka, and the "Hoei-type," where the fault rupture does not extend to Shizuoka. These classifications are proposed by Professor Tetsuzo Seno of the University of Tokyo Earthquake Research Institute [3].
Since the Meio Earthquake in 1498, the earthquake intervals have been considered relatively constant, occurring approximately every 100 years. However, uncertainties exist for earlier intervals, and different theories propose alternating cycles of 100 and 200 years. Some researchers argue for a lack of definitive evidence before the Eichu Earthquake in 1096, while others suggest recurring patterns based on liquefaction traces and geological investigations [14].
The earthquakes that rupture the entire Nankai Trough simultaneously, such as the Hoei Earthquake in 1707, are characterized by significant magnitudes. The Hoei Earthquake is regarded as one of the largest earthquakes in Japan. For example, in Susaki (present-day Susaki City, Kochi Prefecture), the Hoei tsunami reached elevations around 18 meters, while the Ansei tsunami remained at 5-6 meters. The damage reports from the Tosa domain indicate a substantial increase in the number of collapsed and lost houses for the Hoei Earthquake compared to the Ansei Earthquake [20].
Additionally, the occurrence of earthquakes along the Nankai Trough involves the possibility of slip along "spray faults" (fine branching faults that diverge from the main fault). Research has identified numerous spray faults along the Nankai Trough that are believed to have been active in past events [24].
Concerns about the Nankai Trough Megathrust Earthquake intensified after the Great East Japan Earthquake in March 2011. In response, the Japanese government established the "Nankai Trough Megathrust Earthquake Countermeasures Working Group" within the Central Disaster Management Council in July 2012. This working group positioned the anticipated maximum-class earthquake along the Nankai Trough as a "giant disaster surpassing the Great East Japan Earthquake and constituting a national crisis" [33][11].
The Japan Society of Civil Engineers estimated in June 2018 that the total damage over 20 years following the occurrence of the Nankai Trough Megathrust Earthquake could reach a maximum of 1,410 trillion yen [34].
Prediction of Earthquake Occurrence: Using Time Prediction Models: The "time predictable model" suggests a proportional relationship between the displacement caused by an earthquake and the time required for recovery until the next earthquake. Conversely, the "slip predictable model" proposes a proportional relationship between the strain accumulation time since the last earthquake and the displacement caused by the subsequent earthquake. Both models are considered imperfect [35].
Some faults along the Nankai Trough show a tendency to follow the time prediction model, prompting researchers like Kunihiko Shimazaki in 1977 to explore the application of time prediction models to earthquakes along the Nankai Trough [36][37].
Considering the estimated average recurrence interval of 88.2 years for the next M8-class earthquake based on the time prediction model, probabilities were calculated for events within 30 years using the Brownian Passage Time (BPT) distribution [38].
The estimated probability for the occurrence of M8-9 class earthquakes along the Nankai Trough was around 60% - 70% as of January 1, 2013, and increased to approximately 70% - 80% by January 1, 2018 [39][40].
Challenges and Criticisms of Time Prediction Models: Several challenges and criticisms have been raised regarding the application of time prediction models to the Nankai Trough earthquakes. These include uncertainties about the diversity of earthquake source areas along the Nankai Trough and doubts about whether the evaluation based solely on the uplift in Murotsu Port is appropriate. Additionally, the correlation between uplift and the recovery time may not be straightforward, as evidenced by the significant difference between the uplift rate measured by leveling surveys (5-7mm/year) and the rate calculated based on the time prediction model (13mm/year) [44].
Other issues include the potential influence of induced seismicity from other earthquakes, which could constrain the timing of occurrence. Furthermore, concerns about the accuracy of the uplift measurements in Murotsu Port, such as uncertainties in the pre-earthquake water depths and the lack of information on the measurement date, have led to criticisms of the scientific basis of the conclusions [47].
Using Occurrence Intervals Only: Another method involves evaluating earthquake probabilities based solely on occurrence intervals, similar to assessments for other plate boundary earthquakes. However, the choice of including or excluding the Keicho Earthquake in 1605, using all earthquake dates from